In the manufacture of pressure-sensitive recording papers, better known as carbonless copy papers, a layer of pressure-rupturable microcapsules containing a solution of colorless dyestuff precursor is normally coated on the back side of the front sheet of paper of a carbonless copy paper set. This coated backside is known as the CB coating. In order to develop an image or copy, the CB coating must be mated with a paper containing a coating of a suitable color developer, also know as dyestuff acceptor, on its front. This coated front color developer coating is called the CF coating. The color developer is a material, usually acidic, capable of forming the color of the dyestuff by reaction with the dyestuff precursor.
Marking of the pressure-sensitive recording papers is effected by rupturing the capsules in the CB coating by means of pressure to cause the dyestuff precursor solution to be exuded onto the front of the mated sheet below it. The colorless or slightly colored dyestuff, or dyestuff precursor, then reacts with the color developer in the areas at which pressure was applied, thereby effecting the colored marking. Such mechanism for the technique of producing pressure-sensitive recording papers is well known.
Among the well known color developers used on CF record sheets are phenolic-type resins, such as acetylated phenolic resins, salicylic acid modified phenolics and, particularly, novolac type phenolic resins.
Among the well known basic, reactive, colorless chromogenic dye precursors useful for developing colored marks when and where applied to a receiving sheet coated with such color developers are Crystal Violet Lactone (CVL), the p-toluenesulfonate salt of Michler's Hydrol or 4.4'-bis(diethylamino)benzhydrol, Benzoyl Leuco Methylene Blue (BLMB), Indolyl Red, Malachite Green Lactone, 8'-methoxybenzoindoline spiropyran, Rhodamine Lactone, and mixtures thereof.
Microencapsulation has been used in the production of carbonless copy papers for some time. One of the major techniques involves phase separation from an aqueous solution. The complex coacervation process (U.S. Pat. No. 2,800,457 and others) falls into this category. In such a process, a phase separation into a liquid condensed colloid phase and a dilute colloid phase results from two oppositely charged condensed colloids neutralizing each other. Under appropriate conditions, the condensed colloid phase can be induced to first surround and envelope the oil droplets, and then be hardened to form the microcapsules.
Another method of producing CB microcapsules involves polymerization or interfacial crosslinking. Typically a film forming substance, such as an epoxy compound, a polyvalent isocyanate, or a polyacyl halide reactant is dissolved in a hydrophobic oily solution phase and a coreactant such as a polyfunctional amine or a polyvalent hydroxy compound is dissolved in a hydrophilic aqueous solution phase. An emulsion is formed from the two phases, the reactant and coreactant combine in the emulsion, and a wall is formed by interfacial polymerization or interfacial crosslinking around droplets of the oily solution phase (internal phase) to form the microcapsules. See, for example, Kan patent No. 3,432,327 wherein a large number of reactants and coreactants are designated. Generally, the fragile capsules produced by the simple interfacial polymerization/crosslinking are inadequate for the processing procedures needed for preparing carbonless papers, namely, filtration and dispersion into ink vehicles.
A method to improve the strength of capsule walls is shown in U.S. Pat. No. 4,404,251, which discloses printing ink containing microcapsules containing dye precursors. The microcapsules are made by polyaddition of a polyisocyanate and a polyamine, and the aqueous phase may contain protective colloids and emulsifiers.
Reference is also made to U.S. Pat. No. 4,193,889 which discloses microcapsules and a process for the production of microcapsules, the walls of which consist of polycondensates of a film-forming aliphatic polyisocyanate containing at least one biuret group, or polyaddition products thereof, with a chain extending agent. The chain extending agent is preferably either water, a polyol or a polyamine. It is stated in that patent that the so-produced microcapsules have improved toughness, show adequate crosslinking density, and, therefore, are only slightly permeable to easily volatile encapsulated substances.
Another method for dealing with the problems of fragile microcapsules is disclosed in U.S. Pat. No. 4,435,340, wherein an isocyanate is used in the hydrophobic phase and a polyamine, such as a low molecular weight polyamine, is used in the hydrophilic phase. Microcapsules are formed by interfacial polymerization. U.S. Pat. No. 4,356,108 also discloses an encapsulation process by interfacial reaction of an isocyanate and a low molecular weight polyamine.
Finally, an improvement on the interfacial polymerization method of encapsulation is found in my related U.S. Pat. No. 4,898,780, issued Feb. 6, 1990. That improvement involves reacting a crosslinking agent, such as a polyisocyanate, dissolved in the oily solution phase with a polysalt made up of a high molecular weight polyanion, such as casein, and a low molecular weight polycationic polyamine, dissolved in the aqueous solution phase. That process results in microcapsules which are thicker and stronger than microcapsules produced by ordinary interfacial polymerization.
Whichever encapsulation method is used, a problem remains in terms of the ink vehicle and CB coating method. According to the oldest prior art concerning the technology of CB coating, such coating was carried out with an aqueous coating composition over the entire surface of the substrate, as shown in German Offenlegungsschrifts Nos. 1,934,437 and 1,955,542. The process described in these patents has the disadvantage that, following application of the coating composition, the water is evaporated and this requires a considerable input of energy. Additionally, the need for drying requires the use of a complex and expensive apparatus for an aqueous coating composition. Another problem concerns removal of the polluted water which emanates from production and from the purification of the aqueous coating composition.
If volatile organic solvents are used in the production of the coatings, the excess solvent also has to be evaporated in order to dry the coating. This results in the formation of solvent vapors which are particularly dangerous.
There are also numerous known processes for applying coating compositions to a paper substrate. According to the prior art, aqueous or solvent-containing coatings may be applied to a paper substrate by rotogravure or flexoprinting, as shown in U.S. Pat. Nos. 3,016,308 or 3,914,511. These processes also have the disadvantage that the coatings must be subsequently dried. For these reasons, it was proposed, as shown in U.S. Pat. Nos. 3,079,351 and 3,684,549, to take up the microcapsules in waxes and to coat the paper substrate with hot melt systems of this type. See also, U.S. Pat. Nos. 4,112,138 and 4,097,619 which disclose processes for the application of microcapsules to paper by means of a non-aqueous solvent-free hot melt system, or by means of a radiation-curable system. In U.S. Pat. No. 4,161,570 microcapsules are added to a radiation-curable substance without first spray-drying. Although these proposed measures avoid removal of the solvents, the wax-like coating changes the character of the paper because relative large quantities of wax must be applied. Additionally, the melt systems are applied by means of hot carbon printing machines which, although enabling printing, coating with waxes, and finishing to be combined in an online system, always require a separate installation for each process step.
Accordingly, it remains more desirable to use an aqueous-based CB ink if the solids content of that ink is high enough to avoid the problems found with typical aqueous-based CB inks. The major problem with aqueous CB inks is the large drying capacity required. For example, a 45% solids content CB printing ink has a 55% water content which dictates the use of a large amount of energy to dry the coating. In a typical CB coating at 1.25 lbs/ream dry weight (which contains 1.0 lbs/ream capsules), the coating must be applied at 2.78 lbs/ream wet (i.e. 1.25 lbs/ream divided by 0.45). That means that around 1.5 lbs/ream of water must be removed. In addition, when this much water is added to a sheet, numerous controls must be added to the coater to prevent sheet distortions like curl or cockle. Special grades of paper are also required to avoid excessive penetration and web breaks on the coater. The result is a coater and accompanying facilities that are very expensive to build and operate. The centralization of production, due to high capitalization costs, also produces cost inefficiencies in the form of high scrap levels and transportation costs. All of these factors add to the cost of the CB coated sheet.
In order to overcome these problems there have been various proposals for production of high solids content, aqueous CB printing ink. For example, in Jabs U.S. Pat. No. 4,428,978 there is disclosed a process for the production of aqueous suspensions containing from 35 to 60% by weight of microcapsules prepared by interfacial polyaddition from polyisocyanates and H-active compounds, wherein a)an isocyanaurate-modified aliphatic polyisocyanate is used as the polyisocyanate, and b) the suspension is adjusted to a pH value of.ltoreq.7 after the polyaddition reaction. It is disclosed that the suspensions may be converted into agglomerated-free capsule powders, for example by spray drying, or the suspensions may be used directly for the production of aqueous flexograph pastes and concommittently the production of completely or partly coated carbonless copy papers by flexograph process printing. The preferred aqueous solution phase as shown in the examples in Jabs is a polyamine dissolved in water, to which an aqueous acid is added after the polyaddition reaction takes place.
Another patent which discusses high solids content, aqueous-based, CB printing inks is Vassilliades U.S. Pat. No. 4,138,362. Vassilliades discloses producing microcapsules by admixing a water-immiscible, oily material containing an oil-soluble, non-polymeric polyfunctional isocyanate cross-linking agent, and an aqueous solution of a polymeric emulsifying agent in the form of a water-soluble polymer containing recurring --NH.sub.2 or .dbd.NH groups or a water-soluble natural gum containing recurring hydroxy groups. A water-in-oil emulsion is formed and a solid capsule wall is formed by the cross-linking of the emulsifying agent by the isocyanate. When the emulsion contains a natural polymeric emulsifying agent, a viscosity lowering agent in the form of a urea-formaldehyde or alkali metal periodate may be added in order to obtain a higher solids coat weight while at an efficient coating viscosity. The preferred aqueous solution phase as shown in the examples in Vassilliades is an aqueous chitosan or gelatin-mixed solution containing a water-soluble urea-formaldehyde prepolymer. In the case of casein, the capsules produced by the Vassilliades method are extremely poor. The capsules are very fragile, due to a very thin wall, and exhibit poor aging as a result of a steady release of the solutions they contain.
Despite these teachings, processes for printing microcapsules in coating compositions on offset printing machines or even book printing machines were heretofore regarded as unworkable because both in the production of the printing ink and in the distributor rollers of the printing machine and during the printing process, shearing and compressive forces would destroy most of the microcapsules. A process for producing printing inks which may be press applied is described in U.S. Pat. No. 4,404,251, referred to previously, in which formed microcapsules are formulated into the printing ink composition either by stirring the aqueous dispersion of microcapsules directly in the binder and subsequently removing the water in vacuo (the so-called flushing process), or by spray-drying the microcapsules and then adding to the binder. These processes require special equipment and are not entirely satisfactory. In the former process, the hydrophilic nature of the microcapsules may make direct incorporation into the binder very difficult. The spray-drying technique is very costly. Furthermore, during spray-drying some capsules inevitably aggregate which results in a large particle size distribution. The aggregates can easily reach 100 microns or more, and once formed are virtually impossible to break up non-destructively to the capsule. Such large particles are quite unsuitable for inks.
Accordingly, the need remains for an improved process for the production of high solids content, aqueous, CB printing inks and for high solids content, aqueous, CB printing inks which contain microcapsules which are sufficiently strong to be press applied.